Physics Collision

Background

A reconstructed 3DGS scene is a Gaussian point cloud without solid boundaries for walking or collision. The physics collision module turns that space into queryable collision data. Voxels or meshes describe floors, walls, occluders, and walkable regions for walk mode, camera avoidance, area limits, and spatial interaction.

Voxel colliders are produced from 3DGS assets by the Voxel task in splat-transform. The sections below describe the file format and runtime queries. An optional collision.glb mesh may also be provided.

Overview

Voxel data encodes scene occupancy as a sparse voxel octree (SVO) for runtime collision and ray tests:

• Raycast: picking, grounding, line-of-sight checks
• Sphere / capsule: character depenetration

Encoding follows the Laine–Karras layout shared with playcanvas/splat-transform and playcanvas/supersplat-viewer.

Sparse Octree Structure

The octree subdivides a uniform voxel grid and stores only non-empty regions to compress large scenes.

Levels

• treeDepth levels from root to leaf; the finest voxels have edge length voxelResolution.
• Each leaf covers a 4×4×4 block (leafSize = 4), i.e. 64 occupancy bits.

Node types (each uint32 in nodes)

Type	Meaning
Interior node	High 8 bits: childMask (which octants exist); low 24 bits: index of the first child; sibling indices use popcount.
Solid leaf	Value 0xFF000000 (SOLID_LEAF_MARKER): the entire 4×4×4 block is solid.
Mixed leaf	childMask == 0; low 24 bits point to a 64-bit mask in leafData for per-voxel occupancy.

leafData

• Each mixed leaf uses 2 uint32 values (lo, hi), 64 bits total.
• Bit index for (vx, vy, vz) in the block: vx + vy * 4 + vz * 16 (each ∈ [0, 3]).

Traversal

nodes use a compact breadth-first layout; only children in childMask are stored. Queries follow one path from the root for treeDepth levels.

Occupancy at one voxel: world position → (ix, iy, iz) → block (⌊ix/4⌋, …). Descend from nodes[0]:

• Solid leaf: occupied.
• Mixed leaf: test (ix&3, iy&3, iz&3) against leafData.
• Interior node: pick the octant from block coordinates; if missing, empty; else next index is baseOffset plus popcount.

Ray marching: 3D DDA steps through voxels inside the grid bounds; each cell repeats the occupancy query above.

Output files

• voxel-meta.json: grid bounds, voxel size, treeDepth, nodeCount, leafDataCount, etc.
• voxel.bin: binary blob with nodes then leafData (both as uint32 arrays).
• Optional collision.glb mesh.

Raycast & Collision Queries

splat-transform only generates voxel data. Ray tests and depenetration are implemented at runtime after loading the octree.

Raycast

1. Clip the ray to the grid bounds;
2. 3D DDA: step to the next voxel face with the smallest parameter t along X/Y/Z;
3. For each cell, descend the octree and test occupancy;
4. Return the first solid hit; a miss if the ray exits the bounds.

Ray direction need not be normalized. Used for grounding, picking, and short obstacle checks.

Position occupancy

Map a world position to voxel indices and query whether it lies inside geometry, using the procedure above.

Sphere / capsule

For solid voxels in the bounding volume, measure distance from each cell to the sphere center or capsule axis. If less than the radius, accumulate push-out along the shortest separation. Iterate when resolving multi-contact.

Acknowledgements

The voxel pipeline, octree encoding, and file format primarily reference PlayCanvas open-source projects:

Project	URL	Role
playcanvas/splat-transform	https://github.com/playcanvas/splat-transform	Voxelization, nav fill/carve, octree export, collision mesh generation
playcanvas/supersplat-viewer	https://github.com/playcanvas/supersplat-viewer	Runtime voxel collision (raycast, sphere/capsule); see Issues